A number of devices known as liquid crystal devices for controlling transmission or characteristics of light are, of course, known. For convenience such devices will be referred to as light shutters, although the actual "shuttering", i.e., light transmitting and/or blocking functions may be effected by the combination of the liquid crystal device and one or more polarizers and/or other devices associated therewith. Moreover, although a light shutter is preferably a device that provides control of light transmission over a relatively large area as compared to a display in which light may be controlled on a segment by segment (e.g., as in a conventional seven segment display configuration) and/or in a dot by dot arrangement, it is to be understood that the concepts of the invention may be employed in such displays and the like on a scale that is smaller than the relatively large area to which a shutter often relates. Therefore, the term shutter hereinafter is intended to include both such relatively large area devices and smaller area devices that control transmission of light therethrough. Also, the terms liquid crystal cell and liquid crystal device may be used synonymously herein
One exemplary type of liquid crystal devices or light shutters that are known are referred to, for example, as twisted nematic liquid crystal device, cell, shutter, etc. In such a twisted nematic device incident plane polarized light may be transmitted directly through the liquid crystal device without any substantial change in the optical sense when an electric field is applied across the liquid crystal material However, in the absence of the field the liquid crystal causes the plane of polarization to rotate, e.g., by 90 degrees. With the twisted nematic liquid crystal device between a pair of crossed or parallel plane polarizers, then, light transmitted through the assemblage thereof can be controlled as a function of whether or not the electric field is applied across the liquid crystal material in the liquid crystal device.
Another exemplary known liquid crystal device is that sometimes referred to as a birefringent liquid crystal device. In such birefringent liquid crystal devices, due to the birefringent effect and, thus, the retardation of one of the quadrature components of plane polarized light relatively to the other (e.g., the ordinary and extraordinary ray components of such polarized light) as a function of alignment of liquid crystal structure in the liquid crystal device, the direction of plane polarization of light transmitted through such device can be altered, e.g., rotated. Such alteration, or rotation, may be a function of whether or not an electric field input is applied to the liquid crystal material in the device and the magnitude of such an applied field.
An example of a birefringent liquid crystal device is presented in U.S. Pat. Nos. 4,385,806, Re 32,521 and 4,540,243.
The invention as is described further below may be employed with the foregoing and other types of liquid crystal light shutters.
Although the above-described liquid crystal light shutters are of the type which respond to application or not of electric field, it will be appreciated that other types of input may be supplied to the liquid crystal material, such as thermal inputs, magnetic field inputs, etc. consistent with the present invention. Therefore, reference to application of electric field may be considered to include application of another appropriate input that will cause a light transmission controlling function or the like, such as a change in the intensity of light transmitted through the device to which the input is applied.
The invention also employs use of a variable polarizer or controllable polarizer type of device One example of a variable polarizer is a twisted nematic liquid crystal cell which employs pleochroic (or dichroic) dye that aligns with the liquid crystal material in the device in what is known as guest host relationship. In such a variable polarizer, in the absence of an electric field, for example, the liquid crystal material causes alignment of the dye to cause the dye to operate as a plane polarizer. In the presence of such a field the liquid crystal material aligns with the field, for example, and causes the dye also to align therewith so that the dye stops polarization function or ceases to exhibit characteristics of a plane polarizer.
Another type of variable polarizer is disclosed in copending U.S. patent applications Ser. No. 259,951 filed Oct. 19, 1988 and 261,045 filed Oct. 21, 1988. Such a variable polarizer also may be employed in the present invention, as may other types of variable polarizers.
Various devices have been employed in the past to protect the eyes of a person (or to protect some other animate or inanimate object) from particular types of electromagnetic energy or radiation. Exemplary devices are welding helmet devices or goggles to protect the eyes of an individual from the light or other electromagnetic energy emitted during a welding process, goggles or the like to protect the eyes from flash blindness, and/or to protect the eyes, for example, from other sources of bright light, etc. Several exemplary eye protection devices include those disclosed in U.S. Pat. Nos. 3,245,315, Re 29,684, and 4,039,254 and U.K. patent 565,395. Other electromechanically operated light shutter devices are disclosed in U.S. Pat. Nos. 2,548,230 and 3,159,844.
The disclosures of the above-mentioned patents and patent applications are hereby entirely incorporated by reference.
In a conventional liquid crystal device used as a light shutter or for some other purpose, the application of a voltage from which an electric field is derived to the liquid crystal device can be used to cause a dark to clear transition or a clear to dark transition, depending on the orientation of the usual pair of plane polarizers associated with the liquid crystal device, e.g., between which the liquid crystal device usually is positioned. When voltage is increased from a low level (or no applied voltage) to a higher voltage, the liquid crystal directors are driven by the electric field to an "energized" alignment condition, which typically is an alignment that is perpendicular to the surfaces (sometimes referred to as the substrate surfaces) between which the liquid crystal material forming the liquid crystal device is contained. When the voltage is reduced or eliminated, the liquid crystal directors relax to their natural alignment, e.g., generally in parallel with the substrate surfaces or partly in parallel therewith. The voltage applies a much greater force than the relaxation conditions do to the directors, e.g., due to interaction with the surfaces and/or coatings thereon, the tendency of the liquid crystal (especially nematic liquid crystal) to align in a linear fashion, etc., and, therefore, causes the directors to align with respect to the field much faster than the time required for natural relaxation.
In a welding system, and perhaps in other circumstances, it is desirable, sometimes critical, that the switching speed of a shutter device to go from a so-called unprotected mode to a protected mode is particularly very fast, especially in an automatic shutter device. For example, in a welding helmet application, it is important that the switching speed of an automatic liquid crystal shutter device (e.g., controlled to darkened condition automatically in response to sensing of the ignition of the welding arc or the like) be fastest when the liquid crystal device is changing from the clear condition transmitting light to the dark condition to block light. This fast operation helps to assure that the dark condition will exist as promptly as is possible after the welding arc has been ignited. Therefore, it is most appropriate to configure the liquid crystal device, particularly the liquid crystal device in combination with the associated polarizers, in such manner that the dark condition is achieved by applying a voltage. Thus, the clear condition would occur when voltage is removed or is substantially reduced relative to the voltage applied to achieve the dark condition.
Causing the liquid crystal shutter to operate in the manner just described above may tend to cause a different problem. In particular, the clear condition would be achieved by removing the voltage from the liquid crystal device. Unfortunately, though, if there were a system failure, such as a circuit failure or a power supply (e.g., battery) failure, the liquid crystal device would fail to a clear condition creating the risk for eye injury if the failure were to occur during a welding operation or during some other condition during which it is desirable to provide a darkened, i.e., reduced light transmission, condition of the liquid crystal shutter.
One prior technique for preventing such a failure mode condition has used two twisted nematic liquid crystal cells with polarizers aligned in such a way that a failure would result in a partial light blocking condition. Such a system, though, requires a minimum of two liquid crystal cells and three or more polarizers. Liquid crystal systems with this construction contain more components, are more expensive to manufacture, have a narrow dynamic optical range and have limited speed compared to the present invention, which will be described in further detail below. Moreover, the two liquid crystal cell construction of such prior technique has employed plastic polarizers on the outer surfaces of the cell construction which are difficult to clean without damage and can delaminate in harsh environments. The present invention also overcomes these problems of the prior art.